Seat occupancy sensors are nowadays widely used in automotive vehicles to provide a seat occupancy signal for various appliances, such as, e.g. a seat belt reminder, an auxiliary restraint system (airbag), etc. Seat occupancy sensors exist in a number of variants, e.g. based on capacitive sensing, deformation sensing or pressure (force) sensing. Pressure-sensitive seat occupancy sensors have typically been arranged between the foam body of the seat cushion and the seat cover.
The possibility of customization and personalization of the vehicle by the customer is a key selling factor of modern cars. This leads to many different variants of car interiors being offered for one car model. With the increasing number of available options, severe constraints arise concerning the implementation of technical equipment in the vehicle. With seat occupancy sensors arranged between the foam body of the seat cushion and the seat cover, every seat design (leather, cloth, sport, comfort, . . . ) requires specific development effort for the occupant detection system. That induces high development costs and therefore is an unattractive solution for the automotive industry. A problem to be solved is, therefore, to find a sensor solution, which is less influenced by seat design and thus can be used for a greater variety of car seats or even car platforms.
Document DE 197 52 976 A1 discloses a vehicle seat occupancy sensor in the shape of a film-type pressure sensor. The pressure sensor includes a first carrier film, a spacer and a second carrier film, which are disposed on one another in the manner of a sandwich. Contact elements are arranged on the inner surfaces of the carrier films. An opening in the spacer allows the contact elements to get into contact with each other when pressure is applied on the sensor. The pressure sensor is arranged inside a cavity on the bottom side of the foam cushion of the vehicle seat. The pressure sensor is supported by a foam block, which closes the cavity and which rests on the seat pan.
Document DE 20 2010 003 563 U1 discloses a pressure-sensor unit, comprising a film-type pressure sensor. The pressure sensor includes two films maintained at a distance by a spacer material arranged there between. The pressure sensor is disposed on a compressible intermediate layer, which is, in turn arranged on a base plate. Document JP 2011 105278 discloses a seat occupancy sensor comprising a support plate with bearing elements for fixing the support plate on seat cushion suspension springs, wherein the bearing elements define an upper surface from which the top surface of the support plate is recessed. A foam pad carrying a pressure-responsive detector is arranged between the bearing elements on the support plate.
With pressure sensor units arranged on the B-surface of the seat cushion (i.e. on the side facing away from that on which an occupant may seat himself, between the seat cushion and the support thereof), the seat cushion transfers the pressure from the seating surface to the pressure sensor unit. As a matter of fact, the padding of the seat cushion becomes part of the measurement unit. Whereas the production tolerances of film-type pressure sensors may be controlled such that a uniform activation pressure threshold (i.e. the pressure, at which the films of the pressure sensor get into contact with each other) is achieved for the whole production, important investments would be necessary in the seat manufacturers' production in order to guarantee that the foam of every seat cushion has exactly the same thickness and behaves the same way under pressure, in particular, transfers the same amount of pressure to the pressure-sensor unit.